Zirconium alloys are materials known for producing elements which are subjected, in service, to the conditions prevailing inside the core of a nuclear reactor. In particular, such components made of zirconium alloy are used in nuclear reactors cooled by light water, such as a pressurized water reactor (PWR) and a boiling water reactor (BWR). Zirconium alloys are also used in reactors cooled by heavy water, such as a reactor of the CANDU or VVER type. The zirconium alloys are used in particular in the form of tubes in order to make guide tubes for a fuel assembly, fuel rod claddings, which are filled with fuel pellets, or else neutron absorber claddings. Unalloyed zirconium is also used to produce liners for the rod claddings. The zirconium alloys are also used for the manufacture of duplex tubes comprising two co-rolled tubular walls. These alloys are also used in the form of flat products, such as sheets or strips, in order to form structural elements of the fuel assemblies for a nuclear reactor.
In service, all these elements come into contact with water at very high pressure and at high temperature, which may contain additives such as lithium compounds for example, and/or with steam.
It is therefore necessary for the materials used to produce these components to exhibit very high resistance to corrosion by water and steam at high temperature. It is also necessary for such alloys to have very good mechanical properties at high temperature, particularly a very high creep strength.
As mentioned in FR-96/04739, corresponding to EP-0,802,264 various zirconium alloy grades such as Zircaloy 2, Zircaloy 4, zirconium-niobium alloys and other alloys which have been used for the production of fuel assembly components, particularly for light-water-cooled reactors.
In addition to these zirconium alloys, unalloyed or low-alloy zirconium is also used for the manufacture of cladding tubes used in light-water reactors, as the internal lining of fuel elements in order to limit stress corrosion and to increase the resistance to corrosion and to hydriding by water and steam.
All these materials, whether non-alloyed zirconium or zirconium alloys possibly containing addition elements, such as iron, chromium, niobium, tin, nickel, oxygen, vanadium or other elements, have a zirconium content of at least 95% by weight. All these materials will be referred to in the present patent application as “zirconium-based alloys”.
In the abovementioned patent application, it is recommended to add sulphur to the zirconium alloys in an amount of between 8 and 100 ppm by weight. Such sulphur contents of zirconium alloys, which are significantly higher than the residual contents, make it possible in particular to considerably improve the creep strength of the alloys under temperature conditions such as those encountered in a pressurized water nuclear reactor or boiling water nuclear reactor.
It has been shown that very low sulphur contents, of the order of a few ppm, make it possible to considerably increase the creep strength of the zirconium alloys, for example at a temperature of 400° C. Moreover, it has been observed that this beneficial effect of sulphur on the creep strength of zirconium alloys very rapidly reaches a saturation level for relatively low sulphur contents, sulphur contents which are always less than 100 ppm.
In the case of the abovementioned patent application, it was shown that zirconium alloys possibly containing up to 100 ppm sulphur would have not only a markedly improved creep strength but also a resistance to corrosion in water and steam at high temperature which is acceptable or even superior to the corrosion resistance of sulphur-free alloys.
However, this beneficial effect of sulphur on the corrosion resistance has been observed only in certain zirconium alloys and for sulphur contents which are always less than 100 ppm.